feat(v2): ability to add/override theme html metadatas

[slorber]

Motivation

Generic way to add/override some theme metadatas without swizzling the layout

I particular, ability to override the default value we set for twitter:card

module.exports = {
  // ...
  themeConfig: {
    metadatas: [{name: 'twitter:card', content: 'summary'}],
    // ...
  },
};

fixes #3024

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

preview

[docusaurus-bot]

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